The invention relates to a method for operating a brake system of a work machine and a brake system for a work machine.
Work machines, also called construction equipment or construction vehicles, as for instance excavators or wheel loaders are vehicles designed for and used in rough off-road surroundings where trucks or passenger cars are inoperative or, if operated in such surroundings, would probably be damaged when exposed to these rough conditions.
Work machines such as e.g. a wheel excavator, employ parking brakes as well as operating brakes, e.g. a digging brake, which are activated when the work machine is stopped for working operation, e.g. for digging. The parking brakes are activated when the vehicle is stopped in a situation where there is no working operation performed. In conventional work machines, the operating brake pedal has to be pressed down to its fully engaged position in order to engage the operating brakes. To disengage the operating brakes, the operating brake pedal has to be released from its fully engaged position. To release the operating brake pedal, the pedal has to be pressed further down than the fully engaged position with high force before the pedal is released.
EP 0 989 039 B1 discloses a mobile excavator with wheel drive with a braking arrangement comprising operating and parking brakes on each wheel. The operating brakes are actuated by an automatic brake system employing a hydraulic system which comprises a valve controlled by a control unit. The valve activates the brakes independently from an activation of the brake pedal when the automatic brake system is active. For instance, the operating brakes can be activated if the velocity of the work machine is zero and the accelerator pedal is not pressed. The automatic brake system can be turned on or off using an actuator. Inputs are taken which represent the driver intentions, e.g. the position of the brake or accelerator pedal. Outputs are provided for simultaneous operation of the operation brakes and the parking brakes and a blocking device of the pivot axle. The brake system is only operative if the hydraulic pump of the hydraulic system is running.
EP1767421 A1 discloses a brake system where a driver depresses a brake pedal for locking a hydraulic brake device. The brake pedal stays depressed. For releasing the brake pedal, the driver has to depress the brake pedal further to its maximum extend. This requires a high force and is not comfortable for the driver, particularly if during operation of the work machine the operator has to depress and release the brake pedal frequently. During operation of the work machine, the operator has to work both with the brake pedal and the accelerator.
It is desirable to provide a brake system for a work machine, particularly a wheel excavator, which allows for a comfortable and safe operation of the operating brakes. It is also desirable to provide a method for operating a brake system.
According to a first aspect of the invention, a method for operating a brake system for a work machine is proposed, comprising at least one operating brake acting directly or indirectly on at least one ground engagement element of the work machine and at least one operating brake actuator and an accelerator actuator for activating propulsion of the work machine. The operating brake actuator is movable between a fully engaged position and a fully released position. In an activated state of the brake system the operating brake actuator is automatically moved to its fully engaged position when the accelerator actuator is released. Particularly, the at least one operating brake can be engaged and released by hydraulic pressure in a hydraulic circuit which is controllable by a vehicle electronic control unit. More particularly, the operating brake actuator can be mechanically connected to at least one hydraulic cylinder and automatically moved by the piston of the hydraulic cylinder instead of being moved by action of the operator.
According to a further aspect of the invention, a brake system for a work machine is proposed, comprising at least one operating brake acting directly or indirectly on at least one ground engagement element of the work machine and an operating brake actuator which is movable between a fully engaged position and a fully released position and an accelerator actuator for activating propulsion of the work machine. In an activated state of the brake system the operating brake actuator is moved automatically to its fully engaged position when the accelerator actuator is released. In the activated state of the brake system the at least one operating brake is engaged and released by hydraulic pressure in a hydraulic circuit which is controllable by a vehicle electronic control unit.
The operating brake actuator is in the automatic operating brake mode no longer working as an actuator when the operating brakes are actuated automatically but only as an indicator that they have been brought automatically into their engaged position since the system will operate the operating brakes in this mode automatically. According to an advantageous embodiment, the operating brake actuator can operate or can be in operational connection with the operating brakes by activating them in its engaged position and releasing them in its released position.
Favourably, the operator's comfort is improved because an automatic operating brake mode is provided without the operator's interaction which automatically brings the operating brake actuator in its fully engaged position when the accelerator actuator is released and which also automatically releases the operating brake actuator when the accelerator actuator is engaged. For instance, the operator can chose among various modes, such as a working mode, a customer mode, a parking mode, a travelling mode and the like. The working mode and the customer mode can favourably include the automatic operating brake mode. When the respective mode is chosen, e.g. working or customer mode or the like, the automatic operating brake mode can be activated by an actuator, e.g. a switch of a push button or the like. The automatic operating brake mode can be chosen for e.g. performing an operation like digging, which requires securely blocked ground engagement elements, particularly on sloping terrain. The ground engagement elements can be wheels, caterpillar tracks and the like. Preferably, in the automatic operating brake mode the operating brake can be activated as soon as the accelerator actuator is released and the work machine is standing still.
Besides improving the operator's comfort when using the work machine, the status of the operating brake can easily be visualized by the geometrical position of the operating brake actuator. The operator always knows by the actual position of the operating brake actuator if the brake is engaged or not.
The operating brake actuator can be a pedal, a lever, a sliding element or the like. Likewise, the accelerator actuator can be a pedal, a lever, a sliding element of the like. Advantageously, by automatically moving the operating brake actuator in its fully engaged position when the operating brake and the automatic operating brake mode are active, the operator can visually see the state of the operating brake. Thus, the operator can use the work machine alternatively with or without the automatic operating brake mode. In both cases the operating brake actuator is in the same position when the operating brake is engaged. For instance, if the operating brake actuator is a pedal, the pedal is always in a depressed position when the operating brake is engaged.
According to a favourable embodiment of the invention, in the activated state of the brake system the at least one operating brake actuator can be moved automatically to its fully engaged position only if the work machine is not moving or moving with a speed below a predefined speed threshold. Preferably the operating brake actuator can be automatically moved to its engaged position by action of an actuator device. In this case, the vehicle can have a certain “residual” speed of e.g. 1-5 km/h when the operating brake actuator is already allowed to be activated, thereby bringing the machine even faster to a stand still by braking it with the now active operating brakes.
According to a favourable embodiment of the invention, an actuator device, preferably a valve device, can have at least a first operational state and a second operational state. The first operational state corresponds to an engaged position of the accelerator actuator in which the accelerator actuator is engaged. The second operational state corresponds to a released position of the accelerator actuator in which the accelerator actuator is not engaged. In the second operational state of the actuator device, the at least one operating brake can be connectable by the actuator device, preferably valve device, to a high hydraulic pressure which activates the at least one operating brake in order to block the at least one ground engagement element of the work machine in case the at least one operating brake (106L, 106R, 108L, 108R) is connected by the actuator device (30), preferably valve device, to the hydraulic pressure activating said at least one operating brake. Depending on the actual operational state of the accelerator actuator the operating brake can automatically be engaged or released without an action of the operator, except engaging or releasing the accelerator actuator by the operator. Preferably, the actuator device can move the operating brake actuator to its fully engaged position when the device is in its second operational state, without interaction of the operator.
According to a further embodiment of the invention, an element can be provided to move the operating brake actuator back to its fully released position if the actuator device, preferably valve device, is in its first operational state. The element can be e.g. an expansion spring which pushes the operating brake actuator back into its released position when the at least one operating brake is released from the at least one ground engagement element.
Favourably, the actuator device, preferably valve device, in its first operational state can connect the at least one operating brake to a low hydraulic pressure below the high hydraulic pressure activating the at least one operating brake. In a work machine such as a wheel excavator, the low hydraulic pressure level is in the range of not more than a few bar. For instance, in one example the hydraulic pressure at the accelerator actuator can be as low as 2 bar or even less when the accelerator actuator, e.g. an accelerator pedal, is not engaged, i.e. when the actuator device, preferably valve device, is in its second operational state. In this case, the operating brake is subject to a high hydraulic pressure and the operating brake actuator is automatically moved to its fully engaged position. The hydraulic pressure (operating pressure) which activates the at least one operating brake can be several tens of bar, e.g. 65 bar or more.
Preferably, in the first operational state of the actuator device, preferably valve device, the accelerator actuator is in its engaged position, the operating brake actuator is in its released position and the at least one operating brake is connected to a low hydraulic pressure and in its released position. In the second operational state of the actuator device, preferably valve device, the accelerator actuator is in its released position, the operating brake actuator is in its engaged position and the at least one operating brake is connected to a high hydraulic pressure and in its engaged position.
The operating brake actuator can be mechanically or electromechanically or hydraulically locked in its fully engaged position and that said locking preferably can be achieved by a retractable latch which locks the operating brake actuator in the fully engaged position. The operator can still use the work machine without the automatic operating brake mode as it is possible to use a lock for securely fastening the operating brake actuator.
Advantageously, an actuator can be provided for selecting one or more modes of the work machine, wherein at least one mode includes the automatic operating brake mode which activates the at least one operating brake of the brake system.
Preferably a display is provided which indicates the selected mode and/or the status of the automatic operating brake mode. Additionally, information about failure or status of the automatic operating brake mode can be displayed on the display.